To see how much voltage is being drawn, you put the meter probes across the output leads while the LEDs are running. You have it set to voltage and the appropriate range of course, and put the probes on with the right polarity. To see how much current is drawn, you have to put the probes in series with one of the output leads. I never used that function yet so not particularly clear on it myself but that's the general idea. The important thing is to have it set to the right mode. You don't want to put it in series while in voltage mode, for instance, or vice versa, across the leads in current mode. Could blow the meter out. I already knew the current output of my driver so All I tested was voltage and then multiplied to get watts.
Also when you test voltage you may need to hold the probes on there for a few seconds before it shows up. At least on mine it did. Half the time it didn't even show a readout so could just be my meter or how well I held the probes on.